Single-fiber protection in telecommunications networks

ABSTRACT

A solution for detecting and recovering from a failure in a protected single-fiber passive optical network. A detector is used to detect the degradation in power level of optical signals. Furthermore, the invention discloses a variable symmetric split ratio approach to improve the number of splits (e.g. the number of ONUs). A single-fiber passive optical network is disclosed that uses a plurality of passive nodes connected in the optical fiber between the interfaces, wherein in the passive nodes 2-by-2 splitters/combiners are used to couple optical power from and into the optical fiber at a predetermined split ratio.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to passive optical networks, and in particular toprotected passive optical networks.

2. Description of the Related Art

A Passive Optical Network (PON) is a high bandwidth point-to-multipointoptical fiber network. A PON typically consists of an Optical LineTerminal (OLT), which is connected to Optical Network Units (ONU) usingonly cables, optical splitters and other passive components (i.e. nottransmitting signals using electricity). In a PON, signals are routed insuch a way that all signals reach all interim transfer points of thePON.

Passive optical network technology has been considered a very promisingsolution for solving the last-mile problem. Logically a PON has atree-like structure consisting of an optical line terminal, which islocated e.g. in a central office (CO), and a plurality of opticalnetwork units, e.g. 64 ONUs. The PON technology eliminates the need foractive equipment in the field between OLT and ONUs, which are commonlyused in conventional networks. A PON can provide, for example, acapacity of 1 Gbps. A single link failure may result in an intolerabletraffic loss, which indirectly leads to revenue loss. Thus,survivability becomes important especially when a PON is applied in afiber-to-business and cellular-transport (CT) network environment.

Generally, there are two types of survivability architectures: a 1+1architecture and a 1:1 architecture. The 1+1 architecture uses twooverlaid PONs. The traffic is bridged into both a working PON and aprotection PON. Upon receiving a signal in the OLT, the traffic isselected based on signal quality. With this approach, fast protectioncan be achieved. However, in this architecture, no extra traffic can besupported. Compared with the no protection case, it furthermore requiresdouble bandwidth.

In the 1:1 architecture, under normal circumstances, the normal trafficis transmitted over the working PON. Once a failure occurs, the trafficis switched into the protection PON. The protection switching is slowerrelative to that of the 1+1 architecture. However, compared with the 1+1architecture, it can either significantly reduce the spare capacityrequirement or carry extra low priority traffic depending on the networkdesign.

U.S. Pat. No. 6,351,582 discloses one solution for optimizing passiveoptical networks. The passive optical network includes a plurality ofoptical splitters/combiners, each having first and second through portsand at least one drops port. The through ports of the plurality ofsplitters/combiners are concatenated to form a linear arrangement havingtwo end through ports.

FIG. 1 discloses an example of the basic structure of a ring-protectedpassive optical network arrangement described in U.S. Pat. No.6,351,582. In this example, the PON includes two interfaces IF1 and IF2within an OLT unit 110, wherein IF1 operates in active mode and IF2 instandby mode. The PON comprises a plurality of passive nodes 10-13 and15-18, which are preferably splitters/combiners and furthermore aplurality of ONUs 14 and 19. In FIG. 1, splitters/combiners 10, 11, 15and 16 are 1-by-2 or 2-by-1 splitters/combiners that couple opticalpower from and into the optical fiber.

One problem in prior-art solutions and also in the solution disclosed inFIG. 1, is that an optical signal traversing through ringsplitters/combiners 10 and 11 experience optical power losses at twodifferent stages (ring splitters/combiners 10 and 11).

The prior-art passive optical networks involve further problems thathave to be overcome. In an access network, the cost is a major concernsince the number of users in the access network is much less than thatin metro or backbone networks. Furthermore, there exists a problem ofhow to effectively provide protection against a single link failure in aPON based access network without significantly increasing the cost peruser.

A further problem is how to implement fast fault detection in a PON. Yeta further problem is how to fast reroute the affected traffic from theworking OLT to the protection OLT.

A further problem is how to solve the attenuation problem caused byprotection elements.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided asingle-fiber passive optical network which includes a first interfacehaving a first transmitter and a first receiver, a second interfacehaving a second transmitter and a second receiver, an optical fiberconnection between the interfaces and a plurality of passive nodesconnected in the optical fiber between the interfaces. In the passivenodes 2-by-2 splitters/combiners are used to couple optical power to andfrom the optical fiber at a predetermined split ratio.

In one embodiment, a plurality of split ratios is used among the passivenodes.

In a further embodiment, the split ratios are configured to providevariable symmetric split ratios among the passive nodes.

In a further embodiment, the single-fiber passive optical networkfurther includes a detector in the second interface for detecting thedegradation in power level of optical signals received from the firstinterface via the optical fiber connection, and in response todetecting, switching on the second transmitter of the second interface.

According to a second aspect of the invention there is provided a methodof detecting and recovering from a failure in a protected single-fiberpassive optical network including a first interface having a firsttransmitter and a first receiver, a second interface having a secondtransmitter and a second receiver, an optical fiber connection betweenthe interfaces, a plurality of passive nodes between the interfaces, aplurality of optical network units connected to the plurality of passivenodes. The method includes sending optical signals from the firstinterface via the optical fiber connection to the second interface,detecting in the second interface the degradation in power level of theoptical signals from the first interface, and in response to detection,switching on the second transmitter of the second interface.

In one embodiment, the method includes the step of switching on thesecond transmitter of the second interface when the optical power of theoptical signals received with the second receiver drops below apredetermined threshold value.

In a further embodiment, the method further includes the step ofswitching off the second transmitter if the light level increases abovethe predetermined threshold value.

In a further embodiment, the method further includes the step of keepingthe second transmitter switched on if the light level increases abovethe predetermined threshold value.

In a further embodiment, the method further comprises the step ofswitching the second transmitter off and again on in order to verifythat the cable is still broken.

In a further embodiment, the method further comprises the step ofswitching off the second transmitter if detecting abrupt changes in theamount of light after switching on the second transmitter.

In a further embodiment, the method further comprises the step ofswitching on the second transmitter again if the light level decreasesbelow the predetermined threshold value.

In a further embodiment, after detecting in the second interface thedegradation in the power level of the optical signals from the firstinterface, and wherein if the first and second interfaces are located ina single optical line terminal, the method further comprises the stepsof starting in the second interface an auto-discovery process toregister affected optical network units, and in response to the resultof the auto-discovery process, updating an internal routing table of theoptical line terminal and sending the affected downstream traffic of theaffected optical network units using the second interface.

In a further embodiment, after detecting in the second interface thedegradation in the power level of the optical signals from the firstinterface, and wherein if the first and second interfaces are located indifferent optical line terminals, the method further comprises the stepsof starting in the second interface an auto-discovery process toregister affected optical network units, establishing with the secondinterface a dedicated path between the interfaces, sending a notifyingmessage from the second interface to the first interface, the notifyingmessage comprising information about the affected optical network units,forwarding the downstream traffic of the affected optical network unitsfrom the first interface to the second interface via the dedicated pathand forwarding the affected upstream traffic from the affected opticalnetwork units from the second interface to the first interface via thededicated path.

In a further embodiment, the method further comprises the step ofsending from the first interface to the second interface via thededicated path which higher layer addresses are behind the affectedoptical network units.

In a further embodiment, when receiving an upstream packet from thesecond interface, the method further comprises the steps of checkingwhether a packet's destination address is found in a routing table of anoptical line terminal comprising the second interface, and if thedestination address is found in the routing table, sending the packetaccording to the routing table, and if the destination address is notfound in the routing table, sending the packet from the second interfaceto the first interface via the dedicated path.

According to another aspect of the invention there is provided a methodof detecting and recovering from a failure in a protected single-fiberpassive optical network including a first interface having a firsttransmitter and a first receiver, a second interface having a secondtransmitter and a second receiver, an optical fiber connection betweenthe interfaces, a plurality of passive nodes between the interfaces, anda plurality of optical network units connected to the plurality ofpassive nodes. The method includes detecting in the first interface thatsignals are not received from at least one optical network unit, and inresponse to detection, switching on the transmitter of the secondinterface.

In one embodiment, the method further includes switching off thetransmitter of the second interface if the first interface detects thatthe number of optical network units from which signals are not receivedincreases.

In a further embodiment, if the first and second interfaces are locatedin a single optical line terminal, the method further includes startingin the second interface an auto-discovery process to register affectedoptical network units, and in response to the result of theauto-discovery process, updating an internal routing table of theoptical line terminal, and sending the affected downstream traffic ofthe affected optical network units using the second interface.

Additional embodiments of this method are explained hereinafter.

In a further embodiment, if the first and second interfaces are locatedin different optical line terminals, the method further comprises thesteps of establishing with the first interface a dedicated path betweenthe interfaces, sending from the first interface to the second interfacea message ordering to switch on the second transmitter of the secondinterface, starting in the second interface an auto-discovery process toregister affected optical network units, sending a notifying messagefrom the second interface to the first interface, the notifying messagecomprising information about the affected optical network units,forwarding the affected downstream traffic of the affected opticalnetwork units from the first interface to the second interface via thededicated path, and forwarding the upstream traffic from the affectedoptical network units from the second interface to the first interfacevia the dedicated path.

In a further embodiment, the method further comprises the step ofsending from the first interface to the second interface via thededicated path which higher layer addresses are behind the affectedoptical network units.

In a further embodiment, when receiving an upstream packet via thesecond interface, the method further comprises the steps of checkingwhether the packet's destination address is found in a routing table ofan optical line terminal comprising the second interface, and if thedestination address is found in the routing table, sending the packetaccording to the routing table, and if the destination address is notfound in the routing table, sending the packet from the second interfaceto the first interface via the dedicated path.

According to yet another aspect of the invention there is provided aprotected single-fiber passive optical network including a firstinterface having a first transmitter and a first receiver, a secondinterface having a second transmitter and a second receiver, an opticalfiber connection between the interfaces, a plurality of passive nodesbetween the interfaces, a plurality of optical network units connectedto the plurality of passive nodes and a detector in the second interfacefor detecting the degradation in power level of optical signals receivedfrom the first interface via the optical fiber. In response todetecting, the second interface is configured to switch on the secondtransmitter.

Various embodiments of this network are described in detail below.

In one embodiment, the second interface is configured to switch on thesecond transmitter of the second interface when the optical power ofoptical signals received with the second receiver drops below apredetermined threshold value.

In a further embodiment, the second interface is configured to switchoff the second transmitter if the light level increases above thepredetermined threshold value.

In a further embodiment, the second interface is configured to keep thesecond transmitter switched on if the light level increases above thepredetermined threshold value.

In a further embodiment, the second interface is configured to switchthe second transmitter off and again on in order to verify that thecable is still broken.

In a further embodiment, the second interface is configured to switchoff the second transmitter if detecting with the detector abrupt changesin the amount of light after switching on the second transmitter.

In a further embodiment, the second interface is configured to switch onthe second transmitter again if the light level decreases below thepredetermined threshold value.

In a further embodiment, if the first and second interfaces are locatedin a single optical line terminal, the second interface comprisesstarting means for starting an auto-discovery process to registeraffected optical network units and updating means for updating aninternal routing table of the optical line terminal in response to theresult of the auto-discovery process.

In a further embodiment, if the first and second interfaces are locatedin different optical line terminals, the second interface comprisesstarting means for starting an auto-discovery process to registeraffected optical network units, the second interface comprisesestablishing means for establishing a dedicated path between theinterfaces, the second interface comprises sending means for sending anotifying message to the first interface, the notifying messagecomprising information about the affected optical network units, thefirst interface comprises forwarding means for forwarding the downstreamtraffic of the affected optical network units to the second interfacevia the dedicated path, and the second interface comprises forwardingmeans for forwarding the affected upstream traffic from the affectedoptical network units to the first interface via the dedicated path.

In a further embodiment, the first interface further comprises sendingmeans for sending to the second interface via the dedicated path whichhigher layer addresses are behind the affected optical network units.

In a further embodiment, the second interface comprises checking meansfor checking whether a packet's destination address is found in arouting table of an optical line terminal comprising the secondinterface, and if the destination address is found in the routing table,sending means for sending the packet according to the routing table, andif the destination address is not found in the routing table, sendingthe packet from the second interface to the first interface via thededicated path.

In one embodiment, the second interface is configured to switch off thetransmitter of the second interface if the first interface detects thatthe number of optical network units from which signals are not receivedincreases.

In a further embodiment, if the first and second interfaces are locatedin a single optical line terminal, the second interface comprisesstarting means for starting an auto-discovery process to registeraffected optical network units, and updating means for updating aninternal routing table of the optical line terminal in response to theresult of the auto-discovery process.

In a further embodiment, if the first and second interfaces are locatedin different optical line terminals the first interface comprisesestablishing means for establishing a dedicated path between theinterfaces, the second interface comprises starting means for startingan auto-discovery process to register affected optical network units,the second interface comprises sending means for sending a notifyingmessage to the first interface, the notifying message comprisinginformation about the affected optical network units, the firstinterface comprises forwarding means for forwarding the downstreamtraffic of the affected optical network units to the second interfacevia the dedicated path, and the second interface comprises forwardingmeans for forwarding the affected upstream traffic from the affectedoptical network units to the first interface via the dedicated path.

In a further embodiment, the first interface further comprises sendingmeans for sending to the second interface via the dedicated path thehigher layer addresses that are behind the affected optical networkunits.

In a further embodiment, the second interface comprises checking meansfor checking whether a packet's destination address is found in arouting table of an optical line terminal comprising the secondinterface, and if the destination address is found in the routing table,sending means for sending the packet according to the routing table, andif the destination address is not found in the routing table, thesending means configured to send the packet from the second interface tothe first interface via the dedicated path.

According to yet a further aspect of the invention there is provided aprotected single-fiber passive optical network having a first interfacehaving a first transmitter and a first receiver, a second interfacehaving a second transmitter and a second receiver, an optical fiberconnection between the interfaces, a plurality of passive nodes betweenthe interfaces, a plurality of optical network units connected to theplurality of passive nodes, a detecting component for detecting in thefirst interface that signals are not received from at least one opticalnetwork unit, and a sending unit for sending to the second interface amessage to switch on the transmitter of the second interface

According to another aspect of the invention there is provided aninterface arrangement for a protected single-fiber passive opticalnetwork. The arrangement includes a first interface having a firsttransmitter coupled to the fiber for transmitting optical signals on afirst wavelength and a first receiver coupled to the fiber for receivingoptical signals on a second wavelength, a second interface having asecond transmitter coupled to the fiber for transmitting optical signalson a second wavelength and a second receiver coupled to the fiber forreceiving optical signals on a first wavelength. The arrangement furtherincludes a detecting component for detecting that signals are notreceived from at least one optical network unit, and a sending unit forsending to the second interface a message to switch on the secondtransmitter of the second interface.

In one embodiment, the second interface is configured to switch off thetransmitter of the second interface if the first interface detects thatthe number of optical network units from which signals are not receivedincreases.

In a further embodiment, if the first and second interfaces are locatedin a single optical line terminal, the second interface comprisesstarting means for starting an auto-discovery process to registeraffected optical network units, and updating means for updating aninternal routing table of the optical line terminal in response to theresult of the auto-discovery process.

In a further embodiment, if the first and second interfaces are locatedin different optical line terminals the first interface comprisesestablishing means for establishing a dedicated path between theinterfaces, the second interface comprises starting means for startingan auto-discovery process to register affected optical network units,the second interface comprises sending means for sending a notifyingmessage to the first interface, the notifying message comprisinginformation about the affected optical network units, the firstinterface comprises forwarding means for forwarding the downstreamtraffic of the affected optical network units to the second interfacevia the dedicated path, and the second interface comprises forwardingmeans for forwarding the affected upstream traffic from the affectedoptical network units to the first interface via the dedicated path.

In a further embodiment, the first interface further comprises sendingmeans for sending to the second interface via the dedicated path thehigher layer addresses that are behind the affected optical networkunits.

In a further embodiment, the second interface comprises means forchecking whether a packet's destination address is found in a routingtable of an optical line terminal comprising the second interface, andif the destination address is found in the routing table, sending meansfor sending the packet according to the routing table, and if thedestination address is not found in the routing table, said sendingmeans are configured to send the packet from the second interface to thefirst interface via the dedicated path.

s, the second interface comprises sending means for sending a notifyingmessage to the first interface, the notifying message comprisinginformation about the affected optical network units, the firstinterface comprises forwarding means for forwarding the downstreamtraffic of the affected optical network units to the second interfacevia the dedicated path, and the second interface comprises forwardingmeans for forwarding the affected upstream traffic from the affectedoptical network units to the first interface via the dedicated path.

In a further embodiment, the first interface further comprises sendingmeans for sending to the second interface via the dedicated path thehigher layer addresses that are behind the affected optical networkunits.

In a further embodiment, the second interface comprises means forchecking whether a packet's destination address is found in a routingtable of an optical line terminal comprising the second interface, andif the destination address is found in the routing table, sending meansfor sending the packet according to the routing table, and if thedestination address is not found in the routing table, said sendingmeans are configured to send the packet from the second interface to thefirst interface via the dedicated path.

According to yet another aspect of the invention there is provided aninterface arrangement for a protected single-fiber passive opticalnetwork. The interface arrangement includes a first interface having afirst transmitter coupled to the fiber for transmitting optical signalson a first wavelength and a first receiver coupled to the fiber forreceiving optical signals on a second wavelength, a second interfacehaving a second transmitter coupled to the fiber for transmittingoptical signals on a second wavelength and a second receiver coupled tothe fiber for receiving optical signals on a first wavelength. Thearrangement further includes a detector coupled to the fiber fordetecting the degradation in power level of incoming optical signals ofthe first wavelength via the fiber, and in response to detecting, thesecond interface is configured to switch on the second transmitter

Several Embodiments of this aspect are described hereinafter.

In one embodiment, the second interface is configured to switch on thesecond transmitter when the optical power of the received opticalsignals of the first wavelength via the fiber drops below apredetermined threshold value.

In a further embodiment, the second interface is configured to switchoff the second transmitter if the light level increases above thepredetermined threshold value.

In a further embodiment, the second interface is configured to keepingthe second transmitter switched on if the light level increases abovethe predetermined threshold value.

In a further embodiment, the second interface is configured to switchthe second transmitter off and again on in order to verify that thecable is still broken.

In a further embodiment, the second interface is configured to switchoff the second transmitter if the detector detects abrupt changes in theamount of light after switching on the transmitter.

In a further embodiment, the second interface is configured to switchthe second transmitter again on if the light level decreases below thepredetermined threshold value.

In a further embodiment, if the first and second interfaces are locatedin a single optical line terminal, the second interface comprisesstarting means for starting an auto-discovery process to registeraffected optical network units, and updating means for updating aninternal routing table of the optical line terminal in response to theresult of the auto-discovery process.

In a further embodiment, if the first and second interfaces are locatedin different optical line terminals the second interface comprisesstarting means for starting an auto-discovery process to registeraffected optical network units, the second interface comprisesestablishing means for establishing a dedicated path between theinterfaces, the second interface comprises sending means for sending anotifying message to the first interface, the notifying messagecomprising information about the affected optical network units, thefirst interface comprises forwarding means for forwarding the downstreamtraffic of the affected optical network units to the second interfacevia the dedicated path, and the second interface comprises forwardingmeans for forwarding the affected upstream traffic from the affectedoptical network units to the first interface via the dedicated path.

In a further embodiment, the first interface further comprises sendingmeans for sending to the second interface via the dedicated path whichhigher layer addresses are behind the affected optical network units.

In a further embodiment, the second interface comprises checking meansfor checking whether a packet's destination address is found in arouting table of an optical line terminal comprising the secondinterface, and if the destination address is found in the routing table,sending means for sending the packet according to the routing table, andif the destination address is not found in the routing table, said meansfor sending are configured to send the packet from the second interfaceto the first interface via the dedicated path.

The invention has several advantages over the prior-art solutions. Forexample, the invention adopts a single fiber to provide protectionagainst single link failure or single OLT failure. Furthermore, itprovides a solution for implementing fast fault detection and faultisolation. Moreover, a solution is presented for addressing how totransfer affected routing information from a working OLT to theprotection OLT. More particularly, an efficient optical protectionsolution is disclosed, which can be used together with a higher layerprotection solution. Alternatively, the higher layer protection solutioncan be used without the optical protection solution.

The various aspects of the invention are more cost-effective as comparedwith the existing approaches since only a single fiber is used.Furthermore, the embodiments of the invention requires less opticalcomponents than conventional systems.

Lastly, according to the various aspects of the invention, optical powerotherwise lost because of ring protection by using 2-by-2 and 2-by-nsplitters may be saved.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and constitute a part of thisspecification, illustrate embodiments of the invention and together withthe description help to explain the principles of the invention. In thedrawings:

FIG. 1 is a block diagram illustrating a prior-art solution for passiveoptical networks;

FIG. 2 is a flow diagram illustrating a ring-protected PON arrangementin accordance with the invention;

FIG. 3 discloses a transceiver used in prior-art solutions;

FIG. 4 is a transceiver that can be used in PON arrangements inaccordance with the invention;

FIG. 5 is another transceiver that can be used in PON arrangements inaccordance with the invention;

FIG. 6 illustrates a method in accordance with the invention;

FIG. 7 is graphical illustration of power levels of optical signals in afiber cable break situation in accordance with the invention;

FIG. 8 a is a block diagram illustrating the interface arrangement whenthe interfaces are located in a single optical line terminal inaccordance with the invention;

FIG. 8 b is a block diagram illustrating the interface arrangement whenthe interfaces are located in different optical line terminals inaccordance with the invention;

FIG. 8 c is a block diagram illustrating the interface arrangement whenthe interfaces are colocated in a single optical line terminal inaccordance with the invention;

FIG. 8 d is a block diagram illustrating the interface arrangement whenthe interfaces are located in different optical line terminals inaccordance with the invention;

FIG. 9 illustrates an exemplary network topology in accordance with theinvention;

FIG. 10 illustrates an exemplary network topology in accordance with theinvention;

FIG. 11 illustrates an exemplary network topology in accordance with theinvention; and

FIG. 12 illustrates the concept of a variable symmetric split ratio inaccordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 describes a ring-protected PON arrangement configured to serve anumber of subscribers. This arrangement includes an OLT 200 having twointerfaces 217 and 218, each having a transceiver. Interface 217 is anactive interface whereas interface 218 is a standby interface. Thedistribution network in this example includes three 2-by-2 ringsplitters/combiners 20, 26, 29 connected by fiber distribution lines213-216 to form a ring between the two interfaces 217 and 218. Each ringsplitter/combiner 20, 26, 29 is linked to a number of subscriber ONUs25, 28, 212 via intermediate splitters/combiners 22, 23, 24, 27, 210,211. The transmitters of OLT 200 are configured to transmit opticalsignals on a first wavelength (for example 1490 nm). Correspondingly,the receiver of OLT 200 is configured to receive optical signals on asecond wavelength (for example 1310 nm). In order to avoid interference,the transmitter of ONUs 25, 28, 212 are configured to transmit opticalsignals on the second wavelength. Correspondingly, the receivers of ONUs25, 28, 212 are configured to receive optical signals on the firstwavelength.

When optical signals are transmitted from OLT 200 to ONU 25, opticalsignals flow from active interface 217 over fiber distribution line 213to ring splitter/combiner 20. Ring splitter/combiner 20 is configuredsuch that a predetermined amount of the optical power of optical signalsreceived is conveyed via optical medium 214 to ring splitter/combiner 26from which it is transferred via optical medium 215 to ringsplitter/combiner 29, etc. The remaining portion of the optical signalsreceived at ring splitter/combiner 20 is conveyed to the downstreamports of splitter/combiner 23 and 24. The light received by ONU 25 isconveyed via splitters/combiners 22, 24.

In the reverse direction, when optical signals are transmitted from ONU25 to OLT 200, a transceiver at ONU 25 emits an optical signal, which isconveyed via splitters/combiners 24, 22 to ring splitter/combiner 20.Ring splitter/combiner 20 splits the optical signal according to thepredetermined split ratio, and the optical signal is conveyed viaoptical medium 213 to0020active interface 217 whilst the other portionof the optical signal is conveyed via optical medium 214 to ringsplitter/combiner 26 and further via optical medium 215 to ringsplitter/combiner 29 and again via optical medium 216 to standbyinterface 218. In other words, ring splitters/combiners 20, 26, 29 willsplit optical signals from ONUs 25, 28, 212 into different direction ofphysical ring, and signals can reach both active interface 217 andstandby interface 218. In case of one link failure, OLT 200 can stillkeep all ONUs connected.

As illustrated in FIG. 2, there are both 1-by-4 splitters/combiners and2-by-8 splitters/combiners. The 2-by-8 splitters/combiners can be usedto save the number of components without changing optical properties. Ofcourse a single 1-by-4 splitter/combiner may be used if there are lessONUs required to be fed by that particular splitter/combiner.

In one embodiment of FIG. 2, for upstream access (i.e. for traffic fromONUs to the OLT), time division multiple access (TDMA) is used. That is,every ONU 25, 28, 212 can only access the upper link at the time slotthat OLT 200 authorizes it via gate messages, and therefore, nosimultaneous access is allowed.

For comparison purposes, FIG. 3 discloses a transceiver used inprior-art solutions. In FIG. 3, an OLT interface 30 includes atransmitter 32 and a receiver 31. Transmitter 32 is configured totransmit optical signals on a first wavelength (for example 1490 nm).Correspondingly, receiver 31 is configured to receive optical signals ona second wavelength (for example 1310 nm). OLT 30 also typicallyincludes a duplex filter 33 for filtering wavelengths of 1310 nm toreceiver 31.

FIG. 4 discloses one embodiment of a transceiver in accordance with theinvention. In FIG. 4, an OLT interface 40 includes a transmitter 42 anda receiver 41. Transmitter 42 is configured to transmit optical signalson a first wavelength (for example 1490 nm). Correspondingly, receiver41 is configured to receive optical signals on a second wavelength (forexample 1310 nm). OLT interface 40 includes typically also a duplexfilter 43 for filtering wavelengths of 1310 nm to receiver 41.Furthermore, OLT interface 40 includes a 1490 nm detector 44 enablingoptical protection switching. A splitter 45 is configured to split lightto detector 44. Optical protection switching is described in more detailin reference to FIGS. 6 and 7. In the arrangement of FIG. 4, with thelocation of detector 44, one can take advantage of the existing duplexfilter 43.

FIG. 5 discloses another embodiment of a transceiver. In FIG. 5, an OLTinterface 50 includes a transmitter 52 and a receiver 51. Transmitter 52is configured to transmit optical signals on a first wavelength (forexample 1490 nm). Correspondingly, receiver 51 is configured to receiveoptical signals on a second wavelength (for example 1310 nm). OLTinterface 50 includes typically also a duplex filter 53 for filteringwavelengths of 1310 nm to receiver 51. Furthermore, OLT interface 50includes a 1490 nm detector 44 enabling optical protection switching.Optical protection switching is described in more detail with referenceto FIGS. 6 and 7.

In this example detector 54 is not inside the transceiver. If detector54 is positioned as in FIG. 5, one can take advantage of standardtransceiver components.

The arrangement of FIG. 5 includes a 1-by-2 splitter 55 splittingoptical power to detector 54. The splitting ratio is e.g. 90%/10%. A1-by-2 splitter could be replaced by e.g. a wavelength sensitivesplitter, which would drop a small fraction of 1490 nm light only. Theoptical signal to detector 54 is filtered with a filter 56. Filter 56 ise.g. a 1490 band pass filter or a 1310/1490 high pass filter.

FIGS. 6 and 7 describe optical protection switching in accordance withthe invention. Referring to FIG. 6, optical signals are sent from afirst interface of an OLT to a second interface of an OLT, as indicatedin step 60. The interfaces can be in the same optical line terminal, oralternatively, in different optical line terminals. As long as thesecond interface receives optical signals (of a certain wavelength, e.g.1490 nm) from the first interface, it can be assumed that the opticalconnection between the interfaces is undamaged.

Therefore, when the second interface detects 61 the degradation in powerlevel of optical signals from the first interface, it switches 62 on thetransmitter and starts to transmit optical signals towards the firstinterface. Exemplary embodiments for implementing the detectionmechanism are described in reference to FIGS. 4 and 5. In brief, adetector is used to detect whether optical signals transmitted by thefirst interface are received.

FIG. 7 graphically illustrates the optical protection switching inaccordance with the various aspects of invention. In one opticalprotection switching solution, a predetermined threshold value is usedfor determining whether the optical fiber cable between the interfaces(described in FIG. 8) is broken. The interfaces can be located either inthe same OLT or in different OLTs. Normally (point 1), the receivedlight power is above the threshold. The meaning of the threshold valueis that if the power level of the optical signals received at a detectorin the second interface (see FIGS. 4 and 5) drops below the thresholdvalue (point 2), it can be assumed that the fiber cable between the twointerfaces is broken.

In response to the power level drop of the received optical signals, thetransmitter of the second interface is turned on (point 3). From FIG. 7it can be seen that the power level of the received optical signals atthe second interface arises due to reflection of light it transmits butstill stays below the threshold value.

When the fiber cable has been repaired, there is a sudden growth in thepower level of received optical signals (point 4). Because the powerlevel now exceeds the threshold value, it can be assumed that the fibercable has been repaired. Therefore, the transmitter of the secondinterface can be turned off (point 5).

The reflection caused by a broken cable has to be considered indetecting cable repair. In a single fiber ring case there is usuallystill about −10 dB of the transmitted power left when light is receivedat the other end of a repaired fiber ring. This is much more than theabout −20 . . . −30 dB that is normally received from a reflection thatoccurs where a cable is broken. Thus, in most cases it is possible todetermine a threshold value above which it is assumed that the fiber hasbeen repaired. However, sometimes the reflected light can be larger thana useful value of a threshold that is used for protection switching.

FIGS. 6 and 7 depict a solution wherein a simple threshold value can beset to the detector. As mentioned above, there might arise situations,in which the threshold value is exceeded when the transmitter of thesecond interface is turned on even if the fiber cable is still broken.Therefore, this might lead to situations in which the transmitter of thesecond interface is turned off (because the threshold value is exceeded)although the fiber cable is still broken.

According to another embodiment, the aforementioned problem can beavoided. After switching on the second interface (protection interface),it is possible to measure how much light transmitted by the secondinterface is reflected back. The reflected level might be higher thanthe threshold value for assuming cable break, which could trick thesecond interface to believe that cable has been repaired. However,because it is known that the cable is broken (this can be checked byquickly turning the transmitter off and on again) and all light isreflected light, the second interface will remain turned on. As long asthere are no abrupt changes in the amount of light after the secondinterface had been turned on, the second interface knows that the fibercable has not been repaired.

If the power level, however, then changes in some way or another, thesecond interface may assume that the fiber cable has now been repaired,and therefore it can turn off the transmitter. If at this point thepower level of the received optical signals decreases below the originalthreshold value, meaning that the fiber cable is actually still broken,the second interface switches the transmitter quickly back on. Thiscould happen in such a short time that all registered ONUs communicatingthrough the second interface would remain registered.

The above-mentioned approach depends on optical layer detection.However, there is also a higher layer mechanism for detecting a fibercut and enabling the second interface. If the first interface does notreceive signals from one or more ONUs, it considers that these ONUs arelost. The first interface can take actions based on whether it has PONphysical topology information. If the first interface (node) is aware ofthe physical topology information based on the lost ONUs and physicaltopology information, the first interface can diagnose whether thefailure is related to a fiber cut. If all the lost ONUs are behind aspecific splitter on the fiber ring, it is considered that the fiber cutoccurs after that splitter. Then the first interface enables the saidsecond interface.

If physical topology information is not available, and when the firstinterface finds out that it has lost some ONUs, since it cannot figureout where the fault may occur, it will notify the second interface toenable the transmission. If the first and second interfaces arecolocated in the same OLT, the enabling of the second interface issimple to implement. If, however, the interfaces are in different OLTs,in most situations there must be provided a dedicated connection betweenthe interfaces in order to transmit the notification.

When both interfaces work simultaneously, it is possible to find outmore of the type of failure. If ONUs lost from the first interfaceregister to the second interface, the failure is a cable break in thefiber ring. If, on the other hand, the failure is somewhere else and thefiber ring was not broken, turning the second interface on will causeinterference and subsequent loss of even more ONUs. In this case eitherthe first interface or the second interface disables the secondinterface quickly. The approach takes only 1 to 2 ms to detect the fibercut failure along the ring. To avoid frequent switches of the secondinterface due to other failures, such as an ONU failure or the fiber cutfrom a splitter to an ONU, the operator can set a threshold for thenumber of the lost ONUs. Therefore, in preferred embodiments, only whenthe number of lost ONUs is more than a set threshold, the firstinterface will notify the second interface and enable transmission.

After a proper diagnosis of the fiber failure and enabling the secondinterface, an auto-discovery mechanism is performed in the secondinterface to synchronize ONUs with the second interface and measure theround trip delay between the second interface and its attached ONUs.Then the second interface will conduct round trip delay compensation andcan start to send/receive traffic properly.

FIGS. 8 a and 8 b illustrate embodiments for interface arrangements of apassive optical network with the optical layer failure detectionapproach in accordance with the invention. The arrangements of FIGS. 8 aand 8 b enable optical protection switching which was described inreference to FIGS. 6 and 7.

The first interface 86 of an OLT includes a transceiver 87 including atransmitter 890 and a receiver 88. Transmitter 890 is configured totransmit optical signals on a first wavelength (for example 1490 nm).Correspondingly, receiver 88 is configured to receive optical signals ona second wavelength (for example 1310 nm). The first interface 86 of theOLT includes also a duplex filter 89 for filtering wavelengths of 1310nm to receiver 88.

The second interface 80 of an OLT includes a transceiver 81 having atransmitter 85 and a receiver 82. Transmitter 85 is configured totransmit optical signals on a first wavelength (for example 1490 nm).Correspondingly, receiver 82 is configured to receive optical signals ona second wavelength (for example 1310 nm). The second interface 80 ofthe OLT also includes a duplex filter 83 for filtering wavelengths of1310 nm to receiver 82. Furthermore, in a preferred embodiment, thesecond interface 80 includes a 1490 nm detector 84 enabling opticalprotection switching. A splitter 893 is configured to split light todetector 84. Alternatively, detector 84 can be outside transceiver 81instead of being inside transceiver 81.

The arrangements of FIGS. 8 a and 8 b also include an optical medium 892between the two interfaces 80, 86.

The difference between FIGS. 8 a and 8 b is that in FIG. 8 a interfaces80, 86 are colocated at the same OLT (router), as shown in FIG. 9, andin FIG. 8 b interfaces 80, 86 are located at different OLTs (routers),as shown in FIGS. 10 and 11. The routers link a PON to an externalnetwork. The interfaces consist of a physical protocol layer, as well ashigher layer or layers. Thus, routers represent here equipment having ahigher protocol layer or layers. Typical protocol layers are Ethernet,ATM (asynchronous transfer mode), or IP (Internet Protocol) layer or acombination of layers. The following describes how higher layeroperation may be enhanced by signaling between routers to ensure fastprotection switching.

For the colocation case (FIG. 8 a), the node where interfaces 80 and 86are located controls directly both interfaces. When a fiber cable breakoccurs, after detecting and diagnosing which the affected ONUs are, anOLT 91 in FIG. 9 updates its internal routing table without advertisinga PON failure to the rest of the routers in an external network, thuskeeping protection switching local and quick. The trafficdestined/originated to the affected ONUs will be sent/received throughinterface 80 instead of interface 86.

In the non-colocation case (FIG. 8 b) interfaces 86 and 80 are inseparate nodes, which are able to communicate which each other through adedicated path 891. If a fiber cable break occurs in the PON, thetraffic going to the affected ONUs (ONUs that cannot be reached byinterface 86) has to be rerouted to interface 80. In the case of anoptical detection of a failure, as interface 80 detects a cable breakusing detector 84, it activates its transmitter 85, and starts anauto-discovery process to register the affected ONUs. At the same time,interface 80 sends a path establishment request to interface 86, andinterface 86 replies with PATH-ACK. If interface 80 receives thePATH-ACK, the path is setup. Next, interface 80 will send notificationmessage with the affected ONUs' information along the established path891 to interface 86. The path establishment described above can beachieved by using a signaling protocol. For example, an extension ofIETF (Internet Engineering Task Force) developed RSVP (ResourceReservation Set-up Protocol) or CR-LDP (Constraint-based Routing-LabelDistribution Protocol) can be used to carry the needed information. Thepath can be used to exchange information and payload between the tworouters without affecting packet forwarding in the rest of the externalnetwork. Thus, quick protection actions and rerouting of packets betweenthe two routers can be accomplished. After that interface 86 is notifiedof the failure and the affected ONUs so that interface 86 is able tostop sending traffic to the affected ONUs.

In the case where optical detection is not used, the detection of afailure relies on the ability of interface 86 to recognize at a higherlayer that ONUs have been lost from the PON, as described earlier. Thus,the path initialization described above is initiated in interface 86instead of interface 80.

Interface 86 notifies the router that it is attached to, to forward theaffected downstream traffic to interface 80 using dedicated path 891.Interface 80 cannot simply forward all of its received upstream trafficto interface 86. Otherwise, a loop may occur. FIG. 10 shows animplementation for the non-colocation case. There, an interface 103corresponds to interface 86 in FIG. 8 b and an interface 104 correspondsto interface 80 in FIG. 8 b. When a router 102 receives upstream trafficfrom a PON, and if it sends all packets to router 101, then router 101processes these packets as they were originated from itself, somepackets may travel through router 102 to their destinations, thus theloop occurs. To solve this problem, when an upstream packet arrives inrouter 102, router 102 checks the IP packet's destination address fromits routing table, and if the packet's address is in the routing table,router 102 sends the packet according to the routing table. Otherwise,router 102 simply sends the packet to router 101 via the establisheddedicated path between router 101 and 102. The advantage for thisapproach is to keep the routing tables of the external networkunchanged. When protection actions are complete, routers 101 and 102 mayadvertise the new stabilized topology to the external network. Thus, thededicated path between routers 101 and 102, otherwise consuming possiblyexcess bandwidth, is freed from payload traffic. FIG. 9 OLT 91 updatesits internal routing table, the traffic destined/originated to theaffected ONUs will be sent/received through interface 93 instead ofinterface 92.

For achieving the aforementioned functionality for a colocationsituation with optical detection such as that illustrated in FIG. 8 a,interface 80 includes a starting component SM1 for starting anauto-discovery process to register affected optical network units.Interface 80 may further include an updating component UP for updatingan internal routing table of the optical line terminal in response tothe result of the auto-discovery process.

For achieving the aforementioned functionality for a non-colocationsituation with optical detection illustrated in FIG. 8 b, interface 80includes a starting component SM1 for starting an auto-discovery processto register affected optical network units, an establishing component EMfor establishing the dedicated path 891 between interfaces 80, 86, and asending unit SM2 for sending a notifying message to interface 8. Thenotifying message includes information about the affected opticalnetwork units. The interface further includes a forwarding component FW2for forwarding the affected upstream traffic from the affected opticalnetwork units to interface 86 via dedicated path 891. Interface 86includes a forwarding component FW1 for forwarding the downstreamtraffic of the affected optical network units to interface 80 via adedicated path 891.

Furthermore, interface 86 may include a sending unit SM3 for sending tointerface 80 via dedicated path 891 the higher layer addresses that arebehind the affected optical network units. Moreover, Interface 80 mayinclude a checking mechanism CM for checking whether a packet'sdestination address is found in a routing table of an optical lineterminal including interface 80 and a sending unit SM4 for sending thepacket according to the routing table. If the destination address is notfound in the routing table, the sending unit sends the packet frominterface 80 to interface 86 via dedicated path 891.

For achieving the aforementioned functionality for a colocationsituation with only higher layer failure detection illustrated in FIG. 8c, interface 80 includes a starting component SM1 for starting anauto-discovery process to register affected optical network units.Further, an updating portion UP is included for updating an internalrouting table of the optical line terminal in response to the result ofthe auto-discovery process. Correspondingly, interface 86 includes adetecting unit DET for detecting that signals are not received from atleast one optical network unit and a sending unit SM5 for sending tointerface 80 a message to switch on the transmitter for interface 80.

For achieving the aforementioned functionality for a non-colocationsituation with only higher layer failure detection illustrated in FIG. 8d, interface 80 may include a starting component SM1 for starting anauto-discovery process to register affected optical network units, and asending unit SM2 for sending a notifying message to interface 86. Thenotifying message includes information about the affected opticalnetwork units. The interface may further include a forwarding componentFW2 for forwarding the affected upstream traffic from the affectedoptical network units to interface 86 via dedicated path 891. Interface86 includes a forwarding component FW1 for forwarding the downstreamtraffic of the affected optical network units to interface 80 viadedicated path 891. Furthermore, interface 80 includes a checkingmechanism CM for checking whether a packet's destination address isfound in a routing table of an optical line terminal, which consists ofinterface 80 and a sending unit SM4 for sending the packet according tothe routing table. If the destination address is not found in therouting table, sending unit SM4 sends the packet from interface 80 tointerface 86 via dedicated path 891.

Correspondingly, interface 86 includes a detecting unit DET fordetecting that signals are not received from at least one opticalnetwork unit and a sending unit SM5 for sending to interface 80 amessage to switch on the transmitter of interface 80. Furthermore,interface 86 includes a forwarding component FW1 for forwarding thedownstream traffic of the affected optical network units to interface 80via dedicated path 891, a sending unit SM3 for sending to interface 80via dedicated path 891 which higher layer addresses are behind theaffected optical network units, and an establishing mechanism EM forestablishing the dedicated path 891 between interfaces 80,86.

The aforementioned components, units and/or mechanisms may beimplemented with hardware and/or software solutions known to thoseskilled in the art, and therefore specific examples are not described inmore detail.

FIGS. 9-11 illustrate exemplary passive optical network topologies inwhich the invention might be used. Each of the topologies is greatlysimplified and is presented only for demonstrative purposes. FIG. 9describes a loop topology for passive optical networks 90 in which bothinterfaces 92, 93 are in the same OLT 91. Several ONUs 94 are connectedto the optical medium 95. FIG. 10 describes a U chain topology forpassive optical networks 100 in which the first interface 103 is in OLT101 and the second interface 104 in OLT 102. Several ONUs 105 areconnected to the optical medium 106. FIG. 11 describes a loop topologyfor passive optical networks 120 in which the first interface 123 is inOLT 121 and the second interface 124 in OLT 122. Several ONUs 125 areconnected to the optical medium 126.

Optical protection switching provides a fast detection solution fordetecting cable breaks. It was described earlier that there exists aconnection between the two interfaces (other than the optical mediumbetween the interfaces). This enables fast rerouting of information ofaffected ONUs. However, it must be noted that the Internet Protocol (IP)layer is able to learn the altered network topology without directsignaling between the OLTs, but this learning may take many secondswhere packets are lost. This solution is therefore more ineffective thanthe solutions described herein.

Thus, optical protection switching in accordance with various aspects ofthe invention can be used in FIGS. 9 and 10, whereas in FIG. 11 it ismore difficult because the connection between routers 121 and 122 (otherthan optical medium 126) may involve multiple hops and domains.Therefore, a dedicated path between the routers is more difficult to setup, has larger latency, and may be clearly more bandwidth constrainedthan a direct or a few-hop connection. If failure detection is based onthe detector in the second interface, for example in router 102, thesecond interface will activate itself. After some time, routinginformation will be updated using normal IP routing protocols.

If, on the other hand, failure detection is based on the first interfacenoticing a loss of ONUs at a higher layer, now in router 101, router 101must send a notification message to router 102 so that the secondinterface can activate itself. Because in this detection scheme there isa significant probability that the second interface must be deactivatedimmediately upon activation, special care should be taken that messagesbetween the two routers propagate as quickly as the circumstances allow.

FIG. 12 illustrates the concept of a variable symmetric split ratiosolution. The concept is illustrated in a ring-protected passive opticalnetwork comprising an optical line terminal (OLT) 130 including anactive interface 131 and a backup interface 132. The PON includes fourring splitters/combiners 132, 133, 134, 135. The aforementioned elementsare connected to each other as shown in FIG. 12 via an optical medium136. The two interfaces can also be located in different OLTs (not shownin FIG. 12).

Normally one of the two interfaces 131, 132 in OLT 130 is active. Theinterface sends light at one wavelength, normally 1490 nm, and receivesat another, normally 1310 nm. Ring splitters/combiners 132, 133, 134,135 are star points, which divide optical signals to ONUs (not shown inFIG. 12).

In a bi-directional ring (such as a ring-protected passive opticalnetwork) the first drop node in one direction is the last node in theother direction. If the method applicable to a unidirectional fiber wasbe used here (i.e. dropping only a small fraction of optical power fromthe ring in drop nodes near the origin and larger fraction in subsequentdrop nodes), the first drop in the reverse direction would tap morepower than necessary and the last nodes would not receive enough power.Following from that, tapping equal to a fraction of light at each dropnode is a suitable solution. Normally, the same splitting ratio is usedto drop power from the ring in ring splitter/combiners 132, 133, 134,135. However, the number of ONUs may be increased when a variablesymmetric split ratio scheme is used in which a larger proportion oflight is dropped at ring splitters/combiners 132, 133, 134, 135, whichare near to OLT 130, and less power is dropped at ringsplitters/combiners 132, 133, 134, 135, which are midway through thering.

The following table shows how the number of ONUs can be increased by thevariable symmetric split ratio in a system where a power budget of 24 dBis assumed.

Constant split ratio Variable symmetric split ratio 8 per drop, 3 drops8 per drop, 4 drops Drop % dB Drop % dB 20% −19.4 30% −17.6 20% −21.320% −22.0 20% −23.2 20% −23.9 20% −25.1 30% −24.0

A single fiber ring with, for example, 2 km distance between each ringsplitter/combiner 132, 133, 134, 135 is used as the topology. Each ringsplitter/combiner 132, 133, 134, 135 distributes the signal to eightONUs. The dB values indicate received optical power at OLT 130 relativeto the transmitted power at ONUs (upstream direction used in thecalculation is more critical in terms of power budget becauseattenuation is larger at wavelengths usually used by ONUs). In aconstant split ratio, a 20% drop ratio in every ring splitter/combiner132, 133, 134, 135 gives best performance for a three-node systemassuming a stock of splitters with 10% step between each model. However,at a 24 dB power budget there is not enough power at the fourth node.Thus, three nodes is the maximum number of nodes. If on the other hand,a variable symmetric split ratio is used, the fourth node will just fitinto the power budget.

It is obvious to a person skilled in the art that with the advancementof technology, the basic idea of the invention may be implemented invarious ways. The invention and its embodiments are thus not limited tothe examples described above, instead they may vary within the scope ofthe claims.

1. A method of detecting and recovering a failure in a protectedsingle-fiber passive optical network, wherein the protected single-fiberpassive optical network comprises a first interface including a firsttransmitter and a first receiver, a second interface including a secondtransmitter and a second receiver, an optical fiber connecting the firstand second interfaces, a plurality of passive nodes between the firstand second interfaces, and a plurality of optical network unitsconnected to the plurality of passive nodes, and wherein the first andsecond interfaces are co-located at an optical line terminal, the methodcomprising: receiving optical signals from the first interface at thesecond interface via the optical fiber; detecting at the secondinterface a degradation in power level of the optical signals receivedfrom the first interface; synchronizing at least some of the pluralityof optical network units with the second interface; in response to saiddetecting, switching on the second transmitter of the second interface;receiving affected traffic at the second interface via the optical lineterminal, wherein the affected traffic comprises traffic from the firstinterface that was disrupted by a failure associated with thedegradation in power level; and sending the affected traffic from thesecond interface to at least one of the plurality of optical networkunits.
 2. The method according to claim 1, wherein a degradation ofpower level is detected if the power level of the optical signals dropsbelow a predetermined threshold value.
 3. The method according to claim2, further comprising switching off the second transmitter in responseto an increase of the power level above the predetermined thresholdvalue.
 4. The method according to claim 2, further comprising keepingthe second transmitter switched on after the power level increases abovethe predetermined threshold value.
 5. The method according to claim 4,further comprising switching the second transmitter off and on again toverify whether a degradation in power level of the optical signals fromthe first interface is still detected.
 6. The method according to claim2, further comprising switching off the second transmitter in responseto an abrupt change in the power level being detected after switching onthe second transmitter.
 7. The method according to claim 6, furthercomprising switching on the second transmitter again in response to thepower level decreasing below the predetermined threshold value.
 8. Themethod according to claim 1, further comprising conducting round tripdelay compensation at the second interface.
 9. A method of detecting andrecovering a failure in a protected single-fiber passive opticalnetwork, wherein the protected single-fiber passive optical networkcomprises a first interface including a first transmitter and a firstreceiver, a second interface including a second transmitter and a secondreceiver, an optical fiber connecting the first and second interfaces, aplurality of passive nodes between the first and second interfaces, anda plurality of optical network units connected to the plurality ofpassive nodes, the method comprising: receiving optical signals from thefirst interface at the second interface via the optical fiber; detectingat the second interface a degradation in a power level of the opticalsignals received from the first interface; and in response to saiddetecting a degradation in the power level, switching on the secondtransmitter of the second interface; wherein after said detecting thedegradation in the power level, the method further comprises: starting,in the second interface, an auto-discovery process to register affectedoptical network units; in response to a result of the auto-discoveryprocess, updating an internal routing table of an optical line terminalassociated with the second interface; receiving affected traffic at thesecond interface via an optical line terminal, wherein the affectedtraffic comprises traffic from the first interface that was disrupted bya failure associated with the degradation in the power level; andtransmitting the received affected traffic to at least one of theplurality of optical network units.
 10. A method of detecting andrecovering from a failure in a protected single-fiber passive opticalnetwork, wherein the protected single-fiber passive optical networkcomprises a first interface including a first transmitter and a firstreceiver, a second interface including a second transmitter and a secondreceiver, an optical fiber connecting the first and second interfaces, aplurality of passive nodes between the first and second interfaces, anda plurality of optical network units connected to the plurality ofpassive nodes, the method comprising: detecting in the first interfacethat signals are not received from at least one optical network unit; inresponse to said detecting, initiating a switching on of the secondinterface; starting an auto-discovery process to register the at leastone optical network unit; and transmitting disrupted traffic from thefirst interface to the second interface for further transmission to atleast one of the plurality of optical network units, wherein thedisrupted traffic is transmitted from the first interface to the secondinterface across an optical line terminal at which the first and secondinterfaces are co-located.
 11. The method according to claim 10, furthercomprising initiating a switching off of the second transmitter of thesecond interface if the first interface detects that a number of opticalnetwork units from which signals are not received increases.
 12. Amethod of detecting and recovering from a failure in a protectedsingle-fiber passive optical network, wherein the protected single-fiberpassive optical network comprises a first interface including a firsttransmitter and a first receiver, a second interface including a secondtransmitter and a second receiver, an optical fiber connecting the firstand second interfaces, a plurality of passive nodes between the firstand second interfaces, a plurality of optical network units connected tothe plurality of passive nodes, the method comprising: detecting, in thefirst interface, that signals are not received from at least one opticalnetwork unit; and in response to said detecting, initiating a switchingon of the second interface; initiating a switching off of the secondtransmitter of the second interface if the first interface detects thata number of optical network units from which signals are not receivedincreases; triggering an auto-discovery process to register affectedoptical network units, the auto-discovery process comprising updating ofan internal routing table of the optical line terminal; and sendingdisrupted traffic from the first interface to the second interface forfurther transmission to at least one of the plurality of optical networkunits, wherein the disrupted traffic is transmitted from the firstinterface to the second interface across an optical line terminal atwhich the first and second interfaces are co-located.
 13. A protectedsingle-fiber passive optical network, comprising: a first interfaceincluding a first transmitter and a first receiver; a second interfaceincluding a second transmitter and a second receiver; an optical fiberconnecting the first and second interfaces; a plurality of passive nodesbetween the first and second interfaces; a plurality of optical networkunits connected to the plurality of passive nodes; and a detector at thesecond interface for detecting a degradation in power level of opticalsignals received from the first interface via the optical fiber; whereinthe second interface is configured to start an auto-discovery process toregister affected optical network units; and wherein, in response tosaid detecting, the second interface is further configured to: switch onthe second transmitter; receive disrupted traffic from the firstinterface via an optical line terminal at which the first and secondinterfaces are co-located; and transmit the disrupted traffic to atleast one of the plurality of optical network units.
 14. The protectedsingle-fiber passive optical network according to claim 13, wherein thesecond interface is configured to switch on the second transmitter ofthe second interface in response to a determination that the opticalpower level of optical signals received with the second receiver dropsbelow a predetermined threshold value.
 15. The protected single-fiberpassive optical network according to claim 14, wherein the secondinterface is configured to switch off the second transmitter in responseto the power level increasing above the predetermined threshold value.16. The protected single-fiber passive optical network according toclaim 14, wherein the second interface is configured to keep the secondtransmitter switched on after the power level increases above thepredetermined threshold value.
 17. The protected single-fiber passiveoptical network according to claim 16, wherein the second interface isconfigured to switch the second transmitter off and again on to verifywhether a degradation in power level of optical signals received fromthe first interface via the optical fiber is still detected.
 18. Theprotected single-fiber passive optical network according to claim 14,wherein the second interface is configured to switch off the secondtransmitter in response to the detector detecting an abrupt change inthe power level after switching on the second transmitter.
 19. Theprotected single-fiber passive optical network according to claim 18,wherein the second interface is configured to switch on the secondtransmitter again in response to the power level decreasing below thepredetermined threshold value.
 20. A protected single-fiber passiveoptical network comprising: a first interface including a firsttransmitter and a first receiver; a second interface including a secondtransmitter and a second receiver; an optical fiber connecting the firstand second interfaces; a plurality of passive nodes between the firstand second interfaces; a plurality of optical network units connected tothe plurality of passive nodes; and a detector at the second interfacefor detecting a degradation in power level of optical signals receivedfrom the first interface via the optical fiber; wherein, in response tosaid detecting, the second interface is configured to: switch on thesecond transmitter; start an auto-discovery process to register affectedoptical network units; update an internal routing table of the opticalline terminal in response to the result of the auto-discovery process;receive disrupted traffic from the first interface via an optical lineterminal at which the first and second interfaces are co-located; andtransmit the disrupted traffic to at least one of the plurality ofoptical network units.
 21. A protected single-fiber passive opticalnetwork, comprising: a first interface including a first transmitter anda first receiver; a second interface including a second transmitter anda second receiver; an optical fiber connecting the interfaces; aplurality of passive nodes between the first and second interfaces; aplurality of optical network units connected to the plurality of passivenodes; detecting means for detecting that signals are not received atthe first interface from at least one optical network unit; means forstarting an auto-discovery process to register the at least one opticalnetwork unit; sending means for sending, to the second interface, amessage to switch on the second interface; receiving means for receivingdisrupted traffic from the first interface via an optical line terminalat which the first and second interfaces are co-located; andtransmitting means for transmitting the disrupted traffic to at leastone of the plurality of optical network units.
 22. The protectedsingle-fiber passive optical network according to claim 21, wherein thesecond interface is configured to switch off the second transmitter ofthe second interface in response to the detecting means detecting that anumber of optical network units from which signals are not receivedincreases.
 23. A protected single-fiber passive optical networkcomprising: a first interface including a first transmitter and a firstreceiver; a second interface including a second transmitter and a secondreceiver; an optical fiber connecting the first and second interfaces; aplurality of passive nodes between the first and second interfaces; aplurality of optical network units connected to the plurality of passivenodes; detecting means for detecting that signals are not received atthe first interface from at least one optical network unit; and sendingmeans for sending to the second interface a message to switch on thesecond interface; wherein the second interface further comprises:starting means for starting an auto-discovery process to registeraffected optical network units; updating means for updating an internalrouting table of the optical line terminal in response to the result ofthe auto-discovery process; receiving means for receiving disruptedtraffic from the first interface via an optical line terminal at whichthe first and second interfaces are co-located; and transmitting meansfor transmitting the disrupted traffic to at least one of the pluralityof optical network units.
 24. An interface apparatus for a protectedsingle-fiber passive optical network, the interface apparatuscomprising: a first interface including a first transmitter coupled to afiber for transmitting optical signals on a first wavelength, and afirst receiver coupled to the fiber for receiving optical signals on asecond wavelength; a second interface including a second transmittercoupled to the fiber for transmitting optical signals on the secondwavelength, and a second receiver coupled to the fiber for receivingoptical signals on the first wavelength; detecting means for detectingthat optical signals are not received from at least one optical networkunit; means for measuring round trip delays between the second interfaceand optical network units attached to the second interface; means forsynchronizing at least some of the plurality of optical network unitswith the second interface; sending means for sending to the secondinterface a message to switch on the second interface; receiving meansfor receiving disrupted traffic from the first interface at the secondinterface via an optical line terminal at which the first and secondinterfaces are co-located ; and transmitting means for transmitting thedisrupted traffic to at least one of the plurality of optical networkunits.
 25. The interface apparatus according to claim 24, wherein thesecond interface is configured to switch off the second transmitter ofthe second interface in response to the detecting means detecting that anumber of optical network units from which signals are not receivedincreases.
 26. An interface apparatus for a protected single-fiberpassive optical network, the interface apparatus comprising: a firstinterface including a first transmitter coupled to a fiber fortransmitting optical signals on a first wavelength, and a first receivercoupled to the fiber for receiving optical signals on a secondwavelength; a second interface including a second transmitter coupled tothe fiber for transmitting optical signals on the second wavelength, anda second receiver coupled to the fiber for receiving optical signals onthe first wavelength; detecting means for detecting that optical signalsare not received from at least one optical network unit; and sendingmeans for sending to the second interface a message to switch on thesecond interface, wherein the second interface further comprises:starting means for starting an auto-discovery process to registeraffected optical network units; updating means for updating an internalrouting table of the optical line terminal in response to a result ofthe auto-discovery process; receiving means for receiving disruptedtraffic from the first interface via an optical line terminal at whichthe first and second interfaces are co-located; and transmitting meansfor transmitting the disrupted traffic to at least one of the pluralityof optical network units.
 27. An interface apparatus for a protectedsingle-fiber passive optical network, the interface apparatuscomprising: a first interface including a first transmitter coupled to afiber for transmitting optical signals on a first wavelength, and afirst receiver coupled to the fiber for receiving optical signals on asecond wavelength; a second interface including a second transmittercoupled to the fiber for transmitting optical signals on the secondwavelength, and a second receiver coupled to the fiber for receivingoptical signals on the first wavelength; and a detector coupled to thefiber for detecting a degradation in power level of incoming opticalsignals of the first wavelength via the fiber, wherein the secondinterface is configured to synchronize at least some of the opticalnetwork units with the second interface, to measure a round trip delaybetween the second interface and its attached optical network units, andto conduct round trip delay compensation, and wherein in response tosaid detecting, the second interface is further configured to switch onthe second transmitter, to receive disrupted traffic from the firstinterface via an optical line terminal at which the first and secondinterfaces are co-located, and to transmit the disrupted traffic to atleast one of the plurality of optical network units.
 28. The interfaceapparatus according to claim 27, wherein the second interface isconfigured to switch on the second transmitter in response to the powerlevel of the received optical signals of the first wavelength droppingbelow a predetermined threshold value.
 29. The interface apparatusaccording to claim 28, wherein the second interface is configured toswitch off the second transmitter after the power level increases abovethe predetermined threshold value.
 30. The interface apparatus accordingto claim 28, wherein the second interface is configured to keep thesecond transmitter switched on after the power level increases above thepredetermined threshold value.
 31. The interface apparatus according toclaim 30, wherein the second interface is configured to switch thesecond transmitter off and again on to verify that a cable is stillbroken.
 32. The interface apparatus according to claim 28, wherein thesecond interface is configured to switch off the second transmitter inresponse to the detector detecting an abrupt change in an amount oflight after switching on the second transmitter.
 33. The interfaceapparatus according to claim 32, wherein the second interface isconfigured to switch the second transmitter on again in response to theamount of light decreasing below the predetermined threshold value. 34.The interface apparatus according to claim 27, wherein the secondinterface comprises starting means for starting an auto-discoveryprocess to register affected optical network units; and updating meansfor updating an internal routing table of the optical line terminal inresponse to a result of the auto-discovery process.